The invention relates generally to a system and method for operating a turbo-charged compression-ignition engine and, more specifically, to a system and method for substantially eliminating compressor surge and reducing specific fuel consumption and engine exhaust emission in a turbo-charged engine.
In a compression-ignition engine, such as a diesel engine, a fuel injection system injects fuel (e.g. diesel fuel) into compressed air within each of the engine cylinders to create an air-fuel mixture that ignites due to the heat and pressure of compression. Unfortunately, engine efficiency, power output, fuel consumption, exhaust emissions, and other operational characteristics are less than ideal. In addition, conventional techniques to improve one operational characteristic often worsen one or more other operational characteristic. For example, attempts to decrease specific fuel consumption often cause increases in various exhaust emissions. Vehicle exhaust emissions include pollutants such as carbon monoxide, nitrogen oxides (NOx), particulate matter (PM), and smoke generated due to incomplete combustion of fuel within the combustion chamber. The amount of these pollutants varies depending on the fuel-air mixture, compression ratio, injection timing, ambient conditions, and so forth.
Stringent emission regulations have forced emissions from engines to reduce over the years. One of the emissions of concern is nitrogen oxide (NOx) emissions. Exhaust gas recirculation is one of the potential ways to achieve low NOx emission levels. As the percentage of exhaust gas recirculation increases, lower NOx emission levels become achievable. In such a high-pressure exhaust gas recirculation system, the turbo compressor can surge in different situations. In one situation, as the percentage of exhaust gas recirculation increases, the fresh air needed by the engine reduces. This leads to a reduced flow in the compressor, leading the compressor to a surge condition. In another situation, as the throat area of an exhaust gas recirculation mixer disposed proximate to an intake manifold of the engine decreases, the backpressure on the compressor increases, decreasing the pressure ratio across the compressor leading the compressor to a surge condition.
It is desirable to have a system and method for substantially eliminating compressor surge and reducing specific fuel consumption and engine exhaust emission in a turbo-charged engine.